feat: Support promises in privacy namespace

[Rob--W]

This PR adds promise support for the common subset of methods in the privacy namespace that are supported by both Chrome and Firefox.
In the future, we can consider extending the promise support to all objects in the privacy namespace, even if they are only supported by one browser.

Note that CI will currently fail because of bug #81 (which will be fixed by #132).

Supersedes and closes #44.

[rpl]

Hey @Rob--W this looks good to me, there is only a small fix that I'd really like to apply before merging this, that is the small one on the fixture described below (the other two comments are only small super-optional nits).

[rpl]

name doesn't seem to be defined in this background script, this was meant to be the extension name retrieved from the manifest, like in the some of the other test extensions in the test/fixtures dir, e.g.

webextension-polyfill/test/fixtures/runtime-messaging-extension/background.js

Line 1 in d612352

const {name} = browser.runtime.getManifest();

To be fair, it is fine with me if we opt to define name and leave the console.log, but it is also absolutely fine with me if we opt to remove these console.log (from line 1, 4 and 10).

[rpl]

Nit, s/is not/should not/ (only to make it clear what we expect)

[rpl]

Nit, how about rephrase this comment a bit? e.g. "so check the type as determined in the background page"

[rpl]

note to self: this different behavior of set and clear seems something that we should list in the known incompatibilities in the updated README (iirc on Firefox set and clear returns true if the setting has been changes, false otherwise, on the contrary Chrome doesn't pass any parameter to the callback that is called once the operation has been completed).

[Rob--W]

This seems like something that should be documented on MDN: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/types/BrowserSetting

[Rob--W]

Done:

diff --git a/test/fixtures/privacy-setting-extension/background.js b/test/fixtures/privacy-setting-extension/background.js
index 37aab05..ef7bf14 100644
--- a/test/fixtures/privacy-setting-extension/background.js
+++ b/test/fixtures/privacy-setting-extension/background.js
@@ -1,10 +1,5 @@
-console.log(name, "background page loaded");
-
 browser.runtime.onMessage.addListener(async (msg) => {
-  console.log(name, "background received msg", {msg});
   let {method, args} = msg;
   let result = await browser.privacy.services.passwordSavingEnabled[method](...args);
   return {result, type: typeof result};
 });
-
-console.log(name, "background page ready to receive a content script message...");
diff --git a/test/fixtures/privacy-setting-extension/content.js b/test/fixtures/privacy-setting-extension/content.js
index 5a8ce46..78cc4ef 100644
--- a/test/fixtures/privacy-setting-extension/content.js
+++ b/test/fixtures/privacy-setting-extension/content.js
@@ -1,13 +1,13 @@
 test("privacy API should be unavailable in the content script", (t) => {
-  t.deepEqual(browser.privacy, undefined, "browser.privacy is not available in a content script");
+  t.deepEqual(browser.privacy, undefined, "browser.privacy should not be available in a content script");
 });
 
 test("privacy API should support promises", async (t) => {
   async function callSettingAPI(method, ...args) {
     // Invokes: browser.privacy.services.passwordSavingEnabled[method](...args);
     let {type, result} = await browser.runtime.sendMessage({method, args});
-    // In Chrome `undefined` values are serialized to `null`, so use the
-    // pre-determined type instead.
+    // In Chrome `undefined` values are serialized to `null`, so check the type
+    // as determined in the background page.
     return type === "undefined" ? undefined : result;
   }